Open Stack

That is really tough to guess. I don't know any reason why a production
environment would be less susceptible by principle.

During my tests almost all QCOW2 instantiations and launches of
snapshots failed, until applying the fsync fix. Notable exceptions:

 * The cirros and ubuntu original images instantiated right after OpenStack setup, with no other load on the system at all.
 * Windows (due to its size! qemu-img can consume the disk head without catching up, while nova download is far away still writing the disk tail).

Failures of the others were varying, from no boot disk found at all, to
failures during boot. Thus, I wouldn't be too surprised if even a
certain fraction of running instances in production got clipped
unnoticed, but only lost chunks not being read so early, or not at all.

How big that fraction is depends on too many boundary conditions.

What's bothering me most with respect to security is the failure's
stickiness. Once a base image is broken on a compute node, it requires
careful intervention not get promoted into all subsequent
instantiations, also of other users and tenants.

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https://bugs.launchpad.net/bugs/1350766

Title:
  Race condition: compute intermittently corrupts base images on
  download from glance

Status in OpenStack Compute (Nova):
  New
Status in OpenStack Security Advisories:
  Incomplete

Bug description:
  Under certain conditions, which I happen to meet often on my Icehouse
  single node setup, uploaded images or snapshots fail to boot. See also
  https://ask.openstack.org/en/question/42804/icehouse-how-to-boot-a
  -snapshot-from-a-running-instance/

  Reason: When first instantiating a QCOW2 image, it's

  (1)  downloaded as QCOW2 to /var/lib/nova/instances/_base/IMAGEID.part
  (2)  converted to RAW format base /var/lib/nova/instances/_base/IMAGEID.converted using qemu-img

  The step (1) is performed in nova/image/glance.py,
  GlanceImageService.download using buffered IO, which does not
  guarantee the resulting data to be written to disk on file close.
  Consequently, the source image file may not be written completely when
  qemu-img sub-process starts reading in step (2). Whether the result is
  good or bad depends on speed of download, file size, and how quickly
  qemu-image can digest its input.

  Proposed fix: enforce fsync on output File object before returning
  from download. Patch attached.

  Security considerations:

   * Due to the race between resources shared between users and tenants
  (compute node network and filesystem IO) a failure can be triggered
  across tenants, implying the risk of DoS.

   * To make things worse -- with the default setting of not cleaning
  the image cache -- any corrupted image will remain in cache until
  replaced with fresh upload using a new image ID. Affected snapshots
  remain unusable forever, until ex- and re-imported manually under
  better conditions.

   * Base image corruptions here are not detected and cannot be caught.
  Theoretically (a bit esoteric, quite unlikely, but not impossible), an
  attacker might modulate resource usage to precisely create an
  incompletely written image, that boots and runs, but has access
  control information stripped.

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